Sediment-Laden Flow in Open-Channels under Noncapacity and Capacity Conditions

To determine the suspended load component of sediment transport in open-channel flow, the vertical distribution of the concentration of suspended particles is of importance. It is usual to determine this distribution by solving the diffusion-convection equation under appropriate boundary conditions. The exponent in the resulting equation is the Rouse number, defined as z′ = ${\mathit{v}}_{\mathit{s}\mathit{s}}/\beta \overline{\kappa }\mathit{u}*\text{.}$ The $\overline{\beta }-\mathrm{value}$ has been the subject of much research. In natural alluvial channels the sediment-laden flow is usually in capacity (saturation) condition, implying that the flow will charge (saturate) itself with particles available in the bed load and/or on the bed itself. However, simulation of sediment-laden flow in a laboratory flume is achieved typically by externally adding particles to the flow. Consequently, it is not certain that the flow was in capacity condition. The resulting $\overline{\beta }-\mathrm{values}$ are often values for noncapacity conditions. They should not be used for natural alluvial channels, because they are misleading. Reported herein are experiments performed in the laboratory under noncapacity, as well as under capacity, conditions. This study focuses on the experimental determination of the $\overline{\beta }-\mathrm{value}$, which incorporates the ratio of the sediment flux $\overline{{\mathit{c}}_{\mathit{s}}^{\mathit{\prime }}{\mathit{v}}^{\mathit{\prime }}}$ and the momentum flux $\overline{{\mathit{u}}^{\mathit{\prime }}{\mathit{v}}^{\mathit{\prime }}}$, as well as the velocity and concentration profiles, ∂ū/∂y and $\partial \overline{{\mathit{c}}_{\mathit{s}}}/\partial \mathit{y}$, respectively. For experiments with small particles, d₅₀ = 0.135 mm, the $\overline{\beta }-\mathrm{values}$ at capacity condition are smaller than unity; at noncapacity condition, the $\overline{\beta }-\mathrm{values}$ are usually larger than the ones at capacity condition, but all are still smaller than unity.